Nicer landing page

The main page of the documentation should look better and be more useful.
- The logo should be used in suitably big size (different logos for light and dark mode)
- Short intro paragraph that describes Nemo in a few sentences
- A list (that will grow/change later) of illustrated features/use cases (e.g., a screenshot of the online app, or a copy of the performance figure from the KR paper)
- There should be a footer with links to the KBS group page at ICCL, and links to the main nemo github and to the nemo docs github with a call for issues (this footer would be good to have on all pages)

Start at the documentation's main page entry point and inspect how page-wide footers are composed; the issue names no file or test. Done means the landing page includes the requested branding, introduction, illustrated feature list, and links, with the footer available on all pages.

Written by the indexing model from the issue text.